Conventionally, gallium nitride (GaN)-based Group III-V compound semiconductors of cubic zinc-blende crystal type or Wurtzite crystal type are employed for fabricating semiconductor devices, such as a light-emitting device emitting visible light of short wavelength (see, for example, JP-A HEI 2-288388).
Stacked structures for fabricating gallium-nitride-based semiconductor devices are produced by use of a single-crystal substrate composed of a high-heat-resistance aluminum oxide, such as sapphire (α-Al2O3 single crystal) or a garnet solid single-crystal (see, for example, JP-A HEI 7-288231).
The above single-crystal substrate composed of aluminum oxide (e.g., sapphire) has lattice constants considerably different from those of a gallium nitride (GaN)-based semiconductor material.
Thus, the stacked structure for use in fabrication of gallium-nitride-based semiconductor devices is generally formed on a single-crystal substrate by the mediation of a buffer layer. The buffer layer for relaxing mismatch in terms of lattice constants is generally called a “low-temperature-deposited buffer layer,” since the layer has been conventionally formed at relatively low temperature (see, for example, Book written by Isamu AKASAKI, “Group III-V Compound Semiconductors,” published by Baifukan Co., Ltd., 1st edition, Chapter 13, (May 20, 1995)).
The low-temperature-deposited buffer layer is formed from, for example, aluminum nitride (AlN) (see the aforementioned Book). In order to relax lattice constant mismatch with respect to a crystalline substrate, the low-temperature-deposited buffer layer is preferably formed of a polycrystalline material in as-grown state (see, for example, JP-A HEI 2-81484).
Meanwhile, there has been disclosed another technique for forming a low-temperature-deposited buffer layer employing a single-crystal layer formed in as-grown state serving at junction area with respect to a crystalline substrate (see, for example, JP-A HEI 10-321905).
However, the low-temperature-deposited buffer layer formed on a sapphire substrate has a drawback in that the orientation of the single-crystal layer included in the buffer layer is not sufficiently identical on the surface of the sapphire substrate, thereby failing to attain consistent formation, on the substrate, of a GaN-based Group III nitride semiconductor layer having the identically same orientation and excellent single-crystal characteristics.
Thus, an object of the present invention is to provide a specific crystal feature of a single-crystal layer included in a low-temperature-deposited buffer layer such that, on an AlXGaYN (0<X, Y<1, X+Y=1) low-temperature-deposited buffer layer including a single-crystal layer being present in the vicinity of a junction area that is in contact with a sapphire substrate, a gallium-nitride-based semiconductor layer having excellent single-crystal characteristics is suitably formed. Another object of the invention is to provide a stacked structure having a GaN-based nitride semiconductor layer having excellent crystallinity, the layer being formed from a low-temperature-deposited buffer layer including a single-crystal layer having the above crystal feature. Yet another object of the invention is to provide a compound semiconductor device exhibiting excellent characteristics by virtue of employment of the stacked structure.